1. Field of the Invention
The present invention relates to mounting systems for aiming instruments, for example telescopes. More particularly, the invention relates to computerized mounting systems that permit both motorized and manual aiming.
2. Description of Related Art
Some instruments, such as telescopes, need to be aimed. There are a number of benefits that result from automating this aiming process. For example, an automatically aimed telescope might be programmed to target celestial objects that would be too difficult for a user to find. Furthermore, an automatically aimed telescope might be programmed to track such celestial objects over time, thus enabling time-lapse photography.
Generally, such automated systems are calibrated by first establishing the location of the instrument, for example in terms of latitude and longitude, and then aiming the instrument in a known direction, for example toward a well-known and easy-to-locate celestial object, such as the North Star, Polaris. Once the system has been calibrated, any aiming vector for the instrument may be calculated relative to the calibration coordinates.
To implement a set of axes with respect to which such aiming may be referenced and measured, the instrument is mounted for rotation upon one or more shafts (“instrument shafts”) that correspond to each such axis respectively. In the most basic of these systems, each instrument shaft is directly coupled to a motor shaft, which is driven by a motor. This coupling is often implemented through a reduction gearbox to permit the use of a smaller motor. A typical gear reduction is 100:1.
The position of the instrument relative to the calibration coordinates can thus be calculated as a function of the rotation of each instrument shaft. It is well known in the art that such rotation can be measured using shaft encoders, for example optical shaft encoders like those manufactured by US Digital Corporation, 11100 NE 34th Circle, Vancouver, Wash. 98682 USA.
Although it is more direct to measure the rotation of an instrument shaft itself, there is a significant benefit if instead one measures the rotation of the directly coupled motor shaft. Because the motor shaft and the instrument shaft are typically coupled through a reduction gearbox, the motor shaft will rotate many times, for example 100 times, during each rotation of the instrument shaft. Therefore, when an encoder is used to measure rotation of the motor shaft instead of the instrument shaft, the result is a significant increase in precision, typically two orders of magnitude, with any loss in accuracy stemming mainly from whatever backlash might exist in the gearbox.
However, there is a serious shortcoming that results from the direct coupling of an instrument shaft to a motor shaft: the instrument can only be aimed under motor drive. If a user were to apply torque to the instrument shaft in order to manually aim the instrument, that torque would be transferred to the gearbox and the motor and might damage either or both.
One way to overcome this shortcoming has been to insert a normally-engaged clutch between the gearbox and the instrument shaft. When the motor is driven to apply a torque to the motor shaft, that torque is transferred through the gearbox and the clutch to the instrument shaft. However, when a user manually applies a torque to the instrument shaft, the clutch disengages to prevent the torque from acting upon the gearbox, the motor shaft, or the motor.
Although the introduction of this clutch overcomes the restriction against manually aiming the instrument, it does so at a cost. Because the motor shaft is no longer directly coupled to the instrument shaft, the rotation of the motor shaft is no longer an accurate representation of the position of the instrument shaft. In other words, if a user were to manually rotate the instrument shaft, the disengaged clutch would not urge the motor shaft to rotate and so the motor shaft would not rotate in synchronization with the instrument shaft and the instrument.
Conventionally, there have been three solutions to measuring the position of the instrument shaft in clutched systems. First, the encoder has been connected to measure the rotation of the instrument shaft, thereby assuring accuracy but surrendering precision, typically by two orders of magnitude. Second, the encoder has been replaced by a much more precise, and hence much more expensive encoder, which has been connected to measure the rotation of the instrument shaft, ensuring accuracy, precision, and expense. Third, the encoder has been connected to measure the rotation of the motor shaft, ensuring precision; however, to salvage accuracy, the instrument system has had to be recalibrated any time that a torque has been manually applied to the instrument shaft. Clearly, all of these solutions have shortcomings.
Accordingly, what is needed is a better way to measure the position of the instrument in a clutched system: a way that takes advantage of both the precision of motor shaft measurement and the accuracy of instrument shaft measurement.